The present invention relates generally to electrically heated windows in a motor vehicle and, more particularly, to a window assembly and method for heating a movable front door window (side lite) in a motor vehicle wherein the generated heat is concentrated at the portion of the front window through which an operator of the vehicle views an exterior rear view mirror.
Manufacturers of motor vehicles have long recognized the advantages of having systems for defogging and deicing the windows of their vehicles. Previous systems have utilized the heat generated by the internal combustion in the engine to produce warm air which is circulated throughout the passenger compartment of the vehicle to defog and deice the windows. However, such systems experience a substantial time delay while the engine attains a sufficient temperature to heat the air. Furthermore, sufficient air passages must be installed within the vehicle to transport the warm air to the respective windows thus further restricting the amount of interior space available for passengers. As should be readily apparent, these problems are exacerbated the greater the distance from the window to the engine.
In an effort to alleviate these problems, vehicle manufacturers have affixed to the window a conductive film, or a series of small conductors, to generate heat when current flows therethrough. Although predominately used on the stationary front or rear windows of a vehicle, a few systems are known which are adapted to be used on the movable door windows, or side lites.
U.S. Pat. No. 4,410,843 issued to Sauer et al discloses an electrically controlled sliding window and proximity detector which includes a window heating system. The Sauer et al system uses heat generated by current flowing through a series of horizontal, parallel conductors. The series of conductors electrically interconnect a pair of bus bars which are connected to a current source, typically the vehicle battery or alternator. During heating, current generally flows from the current source to one of the bus bars and is then divided between the respective conductors.
As the current flows through the conductors heat is generated due to the resistance of the conductors. The current then returns to the current source via the other bus bar. The Sauer et al invention includes a control circuit for starting and stopping the heating process. A moisture detector monitors the condensation on the window to determine when the window needs defrosted or deiced.
The aforementioned prior art system designs have experienced some significant problems. For example, when deicing or defogging a front door window, it is imperative that the area of the window through which a driver views the exterior rear view mirror be clear. However, the noted prior art systems do not concentrate the heating in this area. For instance, the Sauer et al invention has parallel, horizontal conductors which divide the available heating current based on the respective impedances of the individual conductors. Consequently, the top of the window receives the most current and the highest level of heating. The rear view mirror area of the window does not receive a concentrated heating.
Since the heating current is not concentrated on the rear view mirror area of the window, a weak current source or a higher than normal conductive film resistance could substantially impair the heating of this vital area of the window. In addition, vehicle windows, especially front door windows, regularly have asymmetrical shapes. Thus, horizontal conductors traversing such a window have uneven lengths and unequal impedances causing unwanted irregular heating of the window.
It is thus apparent that there is a need in the art for an improved electrically heated window assembly and method for heating a movable front door window (side lite) of a motor vehicle wherein the heating current, and thus the generated heat, is substantially concentrated at the portion of the front door window through which an operator of the vehicle views an exterior rear view mirror (either operator-side mounted on passenger--side mounted or both) and wherein the heating pattern across the window is substantially uniform.